Handbook on engineering

Front matter

Class Book.

Copig!itW._J50D_.

eOFXRIGHT DEPOSm

Introduction

The object of the writer in preparing this work has been to present to the practical engineer a book to which he can, with confidence, refer to for information regarding every branch of his profession.

Up to the date of the publication of this book, it was impossible to find a plain and practical treatise on the steam boiler, steam pump, steam engine, and dynamo, and how to care for them; electric and hydraulic elevators, and how to care for them ; and all other woi-k that an engineer is apt to come in contact with in his profession.

An experience of over twentj^-tive years with all kinds of engines and boilers, pumps, and all other kinds of machinery, enables the writer to fully understand the kind of information most needed by men having charge of steam engines of every description, and what they should comprehend and employ.

With this object in view, the writer has carefully made note of his past experience, and has also made note of things that came to his notice while visiting different engine rooms, and accordingly, has taken up each subject singly, excluding therefrom, everything not strictly connected with steam engineering.

Particular attention has b en given to the latest improvements in all classes of steam engineering and their proportioning, according to the best modern practice, which, it is hoped, will be of great value to engineers, as nothing of the kind has heretofore been published.

This book also contains ample instructions for setting up, lining, reversing and setting the valves of all classes of engines.

THE AUTHOR.

(iii)

Chapter I. The Elementary Principles Of Electrical Machinery

The operation of electric generators, or dyuainos, as they are ordinarily called, and also that of electric motors, depends upon a simple relation between electricity and magnetism, which will be explained in a simple manner in the following paragraphs.

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A permanent mag-net^ as is well known, is a bar of steel which possesses the power of attracting pieces of iron. These bars may Je made straight, as in P'ig. 1, or in the form of a U, as in Fig. ^, or in any other shape desired. The strength of a permanent nagnet depends upon the kind of steel of which it is made, and

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also upon the temper it is given. Generally speaking, the harder the steel the stronger the magnet. A bar of soft steel, or wrought iron, cannot be made into a permanent magnet of any noticeable strength, but if such a bar is covered with a coil of wire, as shown in Figs. 3 and 4, and a current of electricity is passed through the wire, the bar will be converted into a very strong magnet so long as the current flows. As soon as the electric current stops flowing through the wire, the magnetism of the bar will die out.

Magnets of the last-named type are called electro-magnets, as they do not possess magnet properties except when the electric current flows around them. Electro-magnets, when energized by sufficiently strong electric currents, can be far more powerful than the permanent magnets, and on that account they are used in electric generators and motors. In addition to being stronger magnets, the electro-magnet has the advantage that it can be magnetized and demagnetized almost instantly, by simply cutting off the exciting electric current, and on this account they can be used for parts of electrical machines and apparatus, for which the permanent magnet would be entirely unsuited.

If we test the attractive power of a magnet, we will find that it is greatest at the ends, the force at the middle point being scarcely noticeable. A bar such as Fig. 1 or Fig. 3 might hold a piece of iron weighing several pounds, if presented to either end, while at the middle point, it might not be able to sustain more than an ounce or two. Owing to this fact, the ends are called the poles of the magnet.

If any mag'net is suspended from its center, like a scale beam, and allowed to swing freely, it will be found that it will come to rest in a north and south position, and no matter how violently it may be moved around, it will always come to a state of rest with the same end pointing towards the north. On this account, the ends are called north and south poles, the north pole being the end that points toward the north.

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If two-bar magnets are suspeuded side by side with the north end of one at the top and the north end of the other at the bottom, as is illustrated in Fig. 5, they will attract each other; but if both magnets had the north end at the top, they will push away, as shown in Fig. 6. It is evident that there is a good reason for this difference in action, and this reason we can find out by experiment.

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A magnet needle, s'^ich as is used in mariner's compasses, is simply a small magnet. If we place a magnet bar, as shown in Fig. 7, and then set near to it, in different positions, a compass containing a very small needle, we will find that in these several positions the direction of the needle will be about as is indicated by the small arrows marked h on the curved lines a a; the point of the arrow being the north end, or pole of the needle. The reason why the needle will take up these positions is that the north end of the bar attracts the south end of the needle, and pushes away the north end, just as in Figs. 5 and (I, and the south end of the bar acts in the same way ; so that there is a tug of war going on, so to speak, between the attractions and repulsions of

IIANDHOOK ON ENGINEEKIXG the two ends of the bar upon the two ends of the needle, the result being that the position assumed by the needle is the resultant of these several actions. When the needle is near the

A x« north pole of the bar, its south end is attracted with the greatest force, and when near the south end of the bar, the north end experiences the greatest attraction.

If we were to place the exploring needle in all i30ssible positions near the magnet and trace lines parallel with it, in these positions, we would obtain a large number of curves about the shape of those shown in Fig. 8. As these curves represent the direction into which the magnet needle is turned at the various points in the vicinity of the magnet, they represent the direction in which the combined forces of the two poles act at these two points, hence, these lines are called magnetic lines of force.

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When two mag-nets are suspended as in Fig. 5, the lines of force of both will be in the same direction as is indicated in Fig. 9 by the arrow heads on the curves a a. That this is true can be seen from Fig. 7, in which it will be seen that the arrow heads point toward the south pole and away from the north pole. As the north pole of a magnet has an attraction for the south pole, we can readily see that there is an endwise pull in the lines of force, which tends to make them contract, like rubber bands, hence, we can imagine the lines a a in Fig. 9 to contract and thus draw the two magnet bars together.

The repulsion of the two magnets, when the north poles are at the same end, is illustrated in Fig. 10. Here we see that the lines of force passing on the outside of the bars, as indicated by lines a «, are unobstructed, and can assume their natural posi-

JV tion, but those that pass between the bars, along line c, are pressed out of position. If we assume that the lines of force make an effort to retain their position, like so many wire springs, then we can see that the repulsion is due to the effort that the lines make to assume their natural form in the space between the bars.

Magnetic lines of force have no real existence, they simply indicate the direction in which the force acts, but if we keep this fact in mind, it helps us to understand magnetic actions, if we treat the lines of force as if they were something real. This fact will become more evident as we proceed.

Lines of force always pass from the north to the south pole through the space between these poles, and through the magnet itself, they are assumed to pass from the south to the north pole. The form of the lines of force depends upon the relative position of the north and south poles. In Fig. 1) they are curved, as s ^

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S JV* the magnets are placed side by side, but if the bars were arranged end to end, as in Fig. 11, the lines of force would be straight, as is shown at a. From the north end of the right side magnet, the lines of force would pass in curved line, as in Fig. 10, to the south pole of the magnet on the left side, thus completing the magnetic chain, or circuit, as it is called.

If we take the two magnet bars of Fig. 11 and stand them on end, as in Fig. 12, and suspend a bent wire C in the manner shown, effects can be produced that are interesting and instructive, as they illustrate the principle upon which generators and motors act. The wire C should be journaled at /) I>, so as to swing with as little friction as possible, and its ends are to be connected with a battery B, by means of fine wires a and 6; a switch being provided at c so as to stop the flow of current when desired.

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If the switch c is opened, so that no current flows through C, the latter will not be disturbed, and if we give it a swing, it will oscillate back and forth, like a clock pendulum, and in a few seconds come to rest in the position in which it is shown. If the switch is closed, C will at once swing out of the stream of magnetic lines of force and will remain in that position as long as the current from the battery passes through it. The direction in which C will swing will dei3end upon the direction of the current through it. If with the wires a and b connected with the battery, in the manner shown, the wire C swings to the right side, then if a is connected with e, and h with c?, the direction of swing will be reversed ; that is, C will swing toward the left.

From this experiment we see that the magnetic lines of force can develop a repulsive force against an electric current, and that the direction of the repulsion depends upon the direction of the

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electric current with respect to the direction of the hnes of force. We now desire to know why this repulsion is developed, and this we can ascertain by the following experiments : —

If we affang'e three wires as shown in Figs. 18, 14 and 15, so as to run north and south, the upper end being north, and place over these magnet needles D D D, pivoted at e e e, we will find that if there is no current flowing through the wire, the needle will point tow^ard the north, or be parallel with the wire, as is

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I shown in Fig. 14. If the current runs through the wire from south to north, the north end of the needle will swing to the right, as in Fig. 15, and if the current runs through the wire from north to south, the north end of the needle will swing toward the left, as in Fig. 13. From this we see that an electric current can repel a magnet, and that the direction in which it repels it depends upon the direction of the current.

If we stand the three wires on end, as shown in Figs. 16, 17 and 18, in which A B C represent the wires as seen from above, we will find out more about the relation between electric currents

HANDBOOK OX ENGINEERING, and magnets. If we place four small magnet needles around each one of the wires, as shown at a a a a, we will find that those around the center wire, through which no current flows, will all a

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Fio:. Vi point toward the north, as shown, while those around the wire Fig. 16, through which a current flows upward, that is, away from the center of the earth, will point in a direction opposite to that in which the hands of a clock move; and in wire Fig. 18, in which the electric cun-ent flows down toward the center of the earth, the north ends of all the needles will point in the direction in which the hands of a clock move, that is, just opposite to those in Fio-. 16.

From these actions, we infer at once that when an electric current flows through a wire, the latter becomes surrounded with magnetic lines of force, as is illustrated in Figs. 19 and 20, and that there is a fixed relation between the direction of the cnrrent and that of the lines of force. At A, Fig. 19, the direction of the lines of force is shown for a current moving npward, and at B, Fig. 20, the direction of the lines of force is that due to a cun-ent moving downward through the wire.

Inasmuch as an electric current flowing through a wire is surrounded by magnetic lines of force, we can say that a complete electric current consists of two parts, one the current proper, which traverses the wire, and the other the magnetic casing which envelops the wire. It is the action between the latter part of the current and the lines of force of magnets that develops the current in a generator, or the power in a motor.

With the aid of Figs. 21 and 22, we can now show how the force is developed that thrusts the wire to one side in Fig. 12. The lines of force of the magnet, which constitute what is called the magnetic field, will flow from the north pole at the top to the south pole at the bottom, as is shown in Figs. 21 and 22. If the electric current flows through the wire C from the back toward the front, the lines of force developed around it will have the direction shown in Fig. 21. As lines of force cannot flow in opposite directions in the same space, the lines of the field will swing over to the left side of the wire, but in doing so they will be stretched out of the straight form, and they will also push the lines surrounding the wire out of their central position. Under these conditions, which are illustrated in Fig. 21, the effort made by the field lines to straighten out, together with the effort made by the wire lines to return to the central position, will develop a thrust between the wire and the field, and thus force the former out toward the right side.

If the direction of the current through the wire is reversed so as to flow from front to back, the direction of the lines of force around the wire will be reversed, and will be as in Fig. 22. Under these conditions, the lines of force of the magnetic field will

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swing over to the right side of the wire, and thus the thrust will be in the opposite direction.

Figf. i2 represents the principle of an electric motor in its simplest form, and from it we see that the force that causes the armature to rotate is developed by the repulsion between the magnetism of the field magnet and the magnetism that surrounds the wires wound upon the armature.

It is self-evident that if we undertake to force the wire C through the magnetic field in the opposite direction to that in which it swings, we will have to make an effort to do so ; that is, if we try to move the wire from right to left in Fig. 21, or from left to right in Fig. 22, we will have to apply power. Now nature is a strict accountant and does not allow any power to be lost ; therefore, all the energy we expend in moving the wire through the magnetic field must appear in some other form, and the form in which it appears is as an electric current that is generated in

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the wire. If we were to remove the battery in Fig. 12 aud put in its place au instrument to indicate the presence of a current in the wire, we would find that when we move the latter in the opposite direction to that in which it moves under the influence of the current, we generate a current ; that is, we convert the device into a simple electric generator. If in Fig. 21, we move the wire from right to left, the direction of the current generated in the wire will be the same as that of the current which causes the wire to swing in the opposite direction, that is, from back toward the front. As it is a poor rule that does not work both ways, we would naturally infer that if moving the wire from right to left develops a current from back to front, movement in the opposite direction would develop a current from front to back ; and such is actually the case. This fact can be demonstrated by Fig. 12. Suppose that in this figure we hold C stationary in the central position, and then pass a current through from back toward the front ; this current would exert a force to swing C to the right side. If we release the wire, it will swing to the right and as soon as it begins to move, the current will become weaker, showing that the movement of the wire developed therein a current in the oi^posite direction. If we force the wire over to the left side, the current flowing through it will begin to increase as soon as the wire moves. ^

All the for eg'oing shows us that when a wire is moved through a magnetic held, a current will be generated in it if it forms part of a closed circuit, and it makes no difference whether there is a current already flowing in the wire or not. When the wire is caused to move through the magnetic field by a current flowing through it from an external source, the current developed in it will be in opposition to that which comes from the external source, and, as a consequence, the movement produces an actual reduction of the strength of current flowing through the wire. The stronger the magnetic field and the greater the velocity of the

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wire, the stronger the current generated in opposition to the driving current, and, therefore, the weaker the latter. It is on this account that if a motor is allowed to run free, the faster it runs the weaker the current through it becomes, as the actual current in every case can only be the difference between the main drivingcurrent and the one developed in the wire, which latter runs in the opposite direction.

Chapter II. The Principles Of Electromagnetic Induction

- By EIecttomag:nctic Induction, I mean the induction of electric currents by magnetic action. In tlie preceding chapter it has been shown that if we move a wire through a magnetic Held, an electric current will be generated in it, providing its ends are joined, so as to form a closed circuit. If the ends are not joined, then there will be no current developed, because, an electric current cannot flow except in a closed circuit. When the ends of the wire are not- joined, the movement through the field develops simply an electromotive force. Electromotive force is that force which causes an electric current to flow when there is a circuit in which it can flow. Electromotive force is a long-winded name and on that account it is always abbreviated into e.m.f., so that hereafter when these letters are used, it will be understood that they stand for electromotive force.

Metals and all other substances that allow electric currents to flow through them are called conductors, while glass, mica, wood, paper and many other similar forms of matter that do not allow currents to flow through them are called insulators. The difference between conductors and insulators is only one of degree, for there is no known substance that is an absolute non-conductor of electricity ; that is, a perfect insulator ; and there is no substance that does not resist to some extent the passage of a current — that is, there is no such thing as a perfect conductor. Some substances, like damp paper or wood, which stand midway between good conductors and good insulators, can be regarded as either one or the other, depending upon the service for which they are used. For currents of very low e.m.f., they would be in-

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sulators, but for currents of very high e.m.f., they would be conductors.

The current that will flow through any circuit when impelled by an e.m.f., will have a strength that will depend upon the amount of resistance that opposes its flow. As all conducting materials are not of the same degree of conductivity, their relative values are determined by the amount of resistance they interpose to the flow of the current. The resistance of a conductor is measured in units called ohms ; the strength of current is measured in units called amperes, and the e.m.f. is measured in units called volts. The relation between these units is such that an e.m.f. of one volt will cause a current of one ampere to flow in a circuit having a resistance of one ohm.

When a wire is moved through a magnetic held, the e.m.f. induced in it will be determined by the strength of the field and the velocity with which the wire moves, and will not be affected in any way by the resistance of the circuit of which the wire forms a part. If the resistance is very great, the strength of current generated will be very low, and if the resistance is very low the current will be strong, but in either case the e.m.f. will be the same.

. If movement of the wire in one direction develops an e.m.f. in a given direction through the circuit, then movement of the wire in opposite direction will reverse the direction of the e.m.f. Thus, in Fig. 2o, which represents a magnetic field between the poles N S, if wire a is moved from right to left, it will have induced in it an e.m.f. that will be from back to front, and if the direction of motion of the wire is reversed, the e.m.f. will also be reversed. This will be true whether the wire is near the -^pole or ;6' pole. This being the case, it can be seen that if a represents the end of a wdre moving in the direction of arrow d, and b the end of a wire moving in the opposite direction, the e.m.f. 'sin these two wires will be in opposite directions. The

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direction of the e.m.f. in a will be up from the paper toward the observer, and the direction of the e.m.f. in b will be down through the paper. If these two wires are secured to a shaft placed in the center of the field, then by the continuous rotation

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Fiii of the shaft, the two wires can be made to revolve around the circular path shown.

If these two wires are joined at the ends, as shown in Fig. 24, they will form a closed loop, and although the direction of the induced e.m.f. in the two sides will be opposite, when compared to a fixed point in space, they will be in the same direction so far as the loop is concerned ; that is, both e.m.f. 's will develop currents that will flow through the wire in the same directions.

Returning" to Fig"- 23 it will be noticed that if the wires revolve around the circular path at a uniform velocity, their movement in the direction of line c c will not be uniform, but will be the greatest when the wires are in the position shown, and least, when they cross the line c c. In fact, when the wires cross line c c their motion in the direction of this line will be zero, for this

is the point where the direction of movement reverses. Now, the magnitude of the e.m.f . induced in the wire is proportional to the velocity in the direction of the line c c, hence, when the wires are crossing this line, the e.m.f. will be zero, and when they are one-quarter of a turn ahead of the line, the e.m.f. will be the highest.

In Fig:, 24 we see that in side a, the direction of the current is toward the front, and in b it is the reverse; now, when </ moves through half a turn, it will take the place of b, and the direction of the e.m.f. induced in it will be the same as in b in the figure : that is, it will be the reverse of what it is when passing in front of the pole N. This being the case, it is evident that each time the loop makes a half-revolution, the direction of the current generated in it reverses.

As the loop in Fig. 24 is closed, the current generated in it would be of no practical value, but if we cut the wire at one side and connects the ends with rings as shown at a and b in Fig. 25, then by means of collecting brushes c c we can take the cur-

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rent off through the wires d d. This current, however, would consist of a series of impulses that would flow in opposite directions, each one starthig from nothing and increasing to its greatest

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Y strength when the loop reaches the pqsition shown in the figure, and then declining and reaching the zero value when the loop reaches the vertical position. Such a current is called an alternating current, because it flows first in one direction and then in the opposite direction. All forms of machines that generate currents by electromagnetic induction, develop alternating currents, but in the class of machines known as direct or continuous current, a rectifying device is used w^hich rectifies the current before it reaches the external circuit. This rectifying device is called a commutator, and is illustrated in its simplest form in Fig 26. In this illustration it will be noticed that the ends of the wire, instead of being attached to two independent rings, placed side by side, are secured to two half-rings, placed opposite each other. The brushes c c?, through which the current is taken off, are held stationary; therefore, as can be readily seen, c will make contact with a during one-half of the revolution, and with h during the other half ; and this will also be the case with brush d. Now, as the half-rings with which the brushes are in contact change at each half revolution, it follows that by properly setting the ))rushes, they can be made to pass from one-half ring to the other at the very instant when the direction of the current in the loop reverses, so that through each brush there will be a succession of current impulses, but all in the same direction.

The device shown in Fig. 25 is a perfect alternating current generator, and that shown in Fig. 20 is a perfect direct current generator. In both cases, however, the e.m.f. induced is so low as to be of no practical value. To obtain serviceable machines, capable of developing the e.m.f. and current strength required in practice, it is necessary to provide very strong magnetic fields and to rotate in these a large number of loops of .wire. In order that the operation of such machines may be understood, I will first show how the powerful magnetic fields are obtained.

In Fig"* 27 two wires are shown as seen from the end, these being marked A and B. The lines of force surrounding them are in directions that correspond to opposite directions of current in the wires. In wire ^4, the current flows away from the observer. As can be seen, the lines of force of both wires have to crowd into

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the space between the wires, for on the outside of A the two sets of lines would meet each other head on, and this would also be the case on the right side of wire B. This crowding of the lines of force into the space between the wires causes them to distort from their natural position and instead of being central with the wires, are eccentric to them. If we take a long wire through which a current is flowing and bend it into a loop, we will see that if the current flows out. through one side, it will return through the other side, so that in the two sides of the loop the current will flow in opposite directions. This being the case. Fig. 27 can be regarded as showing the two sides of such a loop, and from it we find that the effect of such a loop is to concentrate within its interior nearly all the lines of force that surround the wire.